1. Field of the Invention
This invention relates to the manufacture of electronic storage media; in particular, this invention relates to polishing hard disk substrates used in computer memory hard disk drives.
2. Description of the Prior Art
Because of their speed and efficiency in storing, retrieving, and manipulating data, electronic computers are a necessity in most business and home environments. The computer's speed depends in part on the time it takes to access data in the computer's memory. When the data are stored on a hard disk, the speed depends on both the rotational speed of the hard disk and the speed of the mechanical arm that positions the read-write head over the hard disk.
Data is written to a hard disk by using a magnetic field generated at the read-write head to align magnetic domains on the hard disk surface in a defined direction. Data is read from a hard disk in a reverse manner, by using magnetic domains to induce a magnetic field in the read-write head; the magnetic field is converted to an electrical signal that is decoded in a known manner. If the read-write head is positioned close to the hard disk surface, a smaller area on the hard disk surface is required to induce a magnetic field on the read-write head, and a smaller area on the hard disk will be magnetized by the read-write head. Accordingly, more data in the form of aligned magnetic domains can be packed onto a hard disk if the read-write head is suspended closer to the surface of the hard disk. Because plated or sputtered magnetic hard disk surfaces can hold smaller magnetic domains, they are preferred over iron oxide-coated surfaces for high-density storage.
FIG. 1 illustrates the layers of materials that make up a hard disk 100. First, a hard disk substrate (also called an aluminum blank) 110 is fabricated to give the hard disk structure its basic form. Substrate 110 is formed with a center hole 110a to accommodate a spindle for rotating the hard disk. Next, a layer 120 of nickel phosphorous approximately 450-550 micro-inches thick is deposited on substrate 110 using electroless plating or other known techniques; nickel phosphorous layer 120 provides a good base for the deposition of a magnetic layer 130, approximately 250-300 .ANG. thick, which is sputter-deposited on top of the nickel phosphorous layer 120. Magnetic layer 130, typically composed of cobalt-nickel, has magnetic domains whose orientation, as discussed above, represents the data stored on the hard disk. Next, a graphite overcoat 140 approximately 50-100 .ANG. thick, which decreases corrosion and wear, is sputter-deposited on magnetic layer 130. Finally, a lubricant 150 is coated over the surface of graphite overcoat 140. Lubricant 150 protects against "stiction," which occurs when the read-write head sticks to the surface of the hard disk. This occurs when the spindle supporting the hard disk has been stationary for an extended time, i.e., the hard disk has not been read from or written to, and the read-write head has been "parked" on the surface of the hard disk. When the hard disk is later rotated, the read-write head sticks to the surface of the hard disk, and when the read-write arm is moved over the surface of the hard disk to a data location, the read-write head can damage both itself and the data stored on the hard disk. Using lubricant 150, when the hard disk begins to rotate, the read-write head easily disengages from the hard disk surface and can soon fly above the hard disk surface.
In one version, disk 100 has an outside diameter of 95 mm and a thickness of 0.8 mm, and center hole 110a has a diameter of 25 mm.
The read-write head can be moved to different locations over the hard disk surface (to access data at these locations) by extending or retracting a mechanical arm so as to move the read-write head in a plane parallel to the hard disk. The combination of one or more disks, read-write heads, arms, and associated circuitry is called a head-disk assembly, or HDA. The read-write heads are suspended above the hard disks as they move to each location where data is to be stored or retrieved; this suspension is created by air flows which are created as the disk rotates. The hard disk is attached to and rotated by a spindle motor, and the rotation creates an aerodynamic lift that suspends the read-write head (formed on an aerodynamically shaped slider) above the hard disk; the faster the hard disk rotates, the higher the read-write head is suspended above the rotating hard disk. Presently, hard disks rotate at approximately 5400 r.p.m. to 7200 r.p.m.; read-write heads, once suspended 30.mu. in. above the surface of the hard disk are now, because of technological advances, suspended approximately 2.mu. in. above the surface of the hard disk.
FIG. 2 shows a portion of a head-disk assembly 200 used in a computer memory system. In FIG. 2, hard disks 210 are attached to and rotate in parallel planes on a spindle 220. Read-write heads 230 are attached to read-write arms 240 which move the read-write heads to locations along the surfaces of hard disks 210.
As discussed above, improvements in data-storage efficiency demand that the read-write head not fly too far above the surface of the hard disk. This low flying height demands that the surface of the hard disk be as smooth as possible, without large peaks and valleys. If the surface of the hard disk contains large peaks, the read-write head will crash into the peaks as the hard disk rotates. When this happens, the read-write head will be irreparably damaged as the result of the "head crash."
From the above discussion, it is clear that a smooth, hard disk surface is critical if the head-disk assembly is to perform properly. A smooth surface is achieved by "planetary polishing machines" which buff and polish the surface of the nickel phosphorous-plated hard disk substrate before the magnetic layer, graphite overcoat, and lubricant are added. To achieve a smooth hard disk surface, the plated substrate has to be polished to a high degree of surface finish. The magnetic layer cannot be polished because doing so would destroy its magnetic properties. The graphite overcoat and the lubricant do not have to be polished because they are thin and conform to the shape of the underlying structure.
The prior art used two or more separate planetary polishing machines: one to rough cut the top and bottom surfaces of the plated substrate (initial polishing), and another to polish the top and bottom surfaces of the plated substrate (final polishing). The first planetary polishing machine secured the plated substrates and introduced a slurry, a semi-liquid combination containing water mixed with an abrasive grit, into a polishing chamber where the plated substrates were held. The hard disk substrates were then rotated with respect to polishing pads located above and below the hard disk substrates to rough cut them and remove asperities. (It does not matter whether the hard disk substrates or the polishing pads rotate, as long as one moves relative to the other.) The first planetary polishing machine polished the largest ripples on the hard disk substrate surface and thus required a slurry having a coarse (large-grained) grit. The second planetary polishing machine polished the top and bottom surfaces of the plated substrates to smaller tolerances, and thus required a slurry having a finer (smaller-grained) grit. A two-step process using both coarse and fine-grit abrasives was necessary to achieve the required tolerances: if only a coarse-grit abrasive were used, the required smoothness could not be achieved; if only a fine-grit abrasive were used, the polishing would take too long and would cause the flatness and roll-off at the edge of the disk to exceed allowed tolerances.
To solve this problem, some prior art polishing processes experimented with using a single coarse-grained slurry in both the initial and final polishing stages, and merely diluted the slurry from the initial polishing stage for use in the final polishing stage. However, these processes failed to achieve the required smoothness.
Other prior art polishing processes used two or more planetary polishing machines, so that slurries would not be mixed. This was necessary because slurries could not be easily removed from the polishing chamber. If any large-grained grit from the initial polishing stage remained in the polishing chamber (in the pores of the polishing pads, for example) during final polishing, the substrate would be too deeply scratched by individual particles of large-grained grit and could not meet the required smoothness tolerances. To overcome this limitation, prior art polishing processes required that each machine use only one slurry. If two slurries were required, two machines were also required. Precautions had to be taken to prevent cross-contamination of the slurries between the two machines. For this reason, the machines were sometimes located in different rooms.
Prior art planetary polishing machines include Speedfam Double Sided Machines, model numbers DSM 16B-5P-II (Planetary Polishing Machine), DSM 18B-5P (Planetary Polishing Machine), DSM 13B-9B (Planetary Disk Polishing System), and DSM 16B-5P (Planetary Polishing System). These machines are not designed to allow different slurries to be used in different stages of the polishing process.
Using two machines to polish hard disk substrates has several drawbacks. For example, because the process uses two machines, it has additional purchase and maintenance costs. Second, the process requires that hard disk substrates be transferred between polishing machines; this additional handling adds time to the polishing process and exposes the hard disk substrates to operator mishandling errors and handling damage.
For the above reasons, there exists a need to reduce the time, the expense, and the exposure of operator error involved in polishing hard disk substrates where a high degree of surface finish is required.